In many different processes today, contaminated liquid streams are generated which contain particulate contaminants. Examples of contaminated liquid streams are liquid coolants from metal working processes containing metal particles, water used to wash workpieces from grinding applications, water containing lint and dirt from clothes washing processes and water containing sludge from waste water treatment processes. Due to environmental and/or economical concerns, it has become desirable to capture, reclaim and re-use these liquid streams after the removal of the particulate contaminants therefrom.
Filter media used in the separation of solids from a liquid typically are required to have six attributes. The filter media should have the ability to bridge solids across its pores within a reasonable time after the beginning of feed thereto, it should exhibit minimum resistance to the flow of filtrate, it should avoid the wedging of particles into its pores, which greatly increases the resistance to flow, it should have sufficient strength to withstand the filtering pressure and mechanical wear, it should be resistant to chemical attack and it should have a smooth surface for easy discharge of the deposited solids.
Of the filter media used today, cotton fabrics are the most common due to their cost and availability in a wide variety of weaves. However, cotton can be attacked by all mineral acids and by many organic acids that can crystallize at operating temperatures. Additionally, cotton fabrics are temperature sensitive and the operating temperature using this media should always be maintained below 200° F.
Filter papers and pulps are often used in processes requiring the retention of very fine solids and for the clarification of liquids containing a small amount of solids. These filter media are available in various degrees of permeability, thickness and strength and some are resistant to strong acids and alkalis. However, they must be well supported in the filter.
Metal fabrics are used in various types of solid-liquid filtration processes and have the advantages of a long life in corrosive and high-temperature environments. However, metal fabrics are expensive, cannot generally be used in the filtration of fine solids and tend to readily clog when soft amorphous particles are filtered. Although metal fabrics can be provided in special weaves which can be used to filter fine solids, these weaves tend to be very expensive and are very difficult to back-flush to remove the solids therefrom.
A wide variety of synthetic fibers is used as filter media in solid-liquid filtration due to their chemical resistance and resistance to bacterial attack. However, synthetic fiber filter media tends to be expensive and plug easily and cannot be used in very high temperature services.
Recently, filter elements having a body made from sintered plastic particles have been used to remove particles from air and gas streams. Herding, et al., U.S. Pat. No. 5,547,481, which is equivalent to DE 4211529A1, discloses a filter element having a permeable porous, substantially inherently stable shaped body made of ultrahigh-molecular, fine-grained polyethylene with an average molecular weight of more than 106 and a further polyethylene component which is fine-grained in the initial state and has an average molecular weight of less than 106. A fine-pore coating of small polytetrafluoroethylene is provided on the afflux side of the shaped body and fills at least part of the depth of pores present on the afflux surface of the shaped body. This reference states that it is critical that the coating fill at least a considerable depth of the surface pores. Although Herding, et al. states that the coated shaped body is suitable in general for separating particles from liquid or gaseous media to be filtered, the only contemplated and actual use of the filter element is the separation of solid particles from air and gaseous media. Herding, et al., U.S. Pat. No. 6,331,197, also discloses a sintered plastic particle filter element which is used to remove particulate materials from a gaseous media. The filter element of this reference avoids the mounting and reinforcing steel support members that are required in U.S. Pat. No. 5,547,481 by manufacturing the filter element by uniting two halves of sintered-together plastic particles, each containing a side wall. However, this filter also is only contemplated for use in separating particles from a gaseous stream.
Accordingly, it is an object of the present invention to provide an improved method and system for separating particulate solids from a contaminated liquid medium containing the particulate solids and in which a filter medium is used which has such a high durability it can be made permanent, shows a high ability in separating particulate solid contaminants from a liquid medium, can be used in a wide variety of services and is relatively inexpensive.